The invention relates to control valves especially adapted for absorption dynamometers such as so called eddy brakes. The discharge conduit is connected to the housing of the dynamometer. An adjustment valve may be arranged between the exit port of the brake and the valving mechanism proper.
It is known to vary the exit openings of the discharge conduit connected to hydraulic dynamometer brakes. With the aid of such variation of the exit area, it is possible to vary the filling condition in the brake independently of the pressure in the brake. However, it has been found that dynamometer brakes controlled in this manner exhibit differing control characteristics in different points of operation, especially when the loading or the load removal takes place rapidly. The valve means employed heretofore, for the just described purposes are normally of the slide valve type whereby the slide proper is subject to the pressure resulting from the dynamic flow conditions. Thus, it is necessary to employ relatively large adjustment mechanisms for controlling the adjustment valve.
From German Pat. No. 728,010 and German Patent Publication No. 1,011,171 it is known to vary the size of the exit area for the work liquid flowing out of a dynamometer brake, by means of a valve which is connected to a piston. The piston is subject to the feed pressure of a rotary pump which in turn is driven by the shaft of the brake. However, the valve structures required for this type of apparatus are relatively complicated and expensive.
Further difficulties are encountered due to the pressure delivered by the rotary pump. If the rotary pump is dimensioned in approximation to the blade or vane arrangement of the dynamometer brake, the rotary pump will deliver a very low pressure at low rpms. Accordingly, it is necessary to provide for special structural features for achieving the required brake pressure. For example, it may be necessary to subject additional piston surfaces with a pressure effective in the same direction and which is larger than the pressure delivered by the rotary pump. On the other hand, at large rpms the rotary pump produces a very high pressure which is substantially above the pressure required for the valve adjustment.
According to another known control mechanism disclosed in German Pat. No. 744,509 the valve located in the discharge conduit of the dynamometer brake is connected to a displacement member, for example a piston, and subjected to the pressure delivered by a displacement pump which is driven by the rpm of the dynamometer brake. In this prior art device, the displacement member such as a piston or membrane is subjected to an oil pressure in a closed oil circulatory system which is separate from the working liquid, such as water used in the hydraulic brake. The size of the exit area is controlled or rather varied by the separate oil pressure. Accordingly, it is necessary to provide a seal between the valve and the displacement member. The resulting friction causes an operation of the control mechanism which is not precise and in addition, is subject to hysteresis deficiency. Besides, the structure of the control mechanism is rather complicated and expensive.
In view of the above difficulties, it has been preferable heretofore to employ electronic control devices for the adjustment of dynamometer brakes such as water eddy brakes. However, especially in connection with relatively small dynamometer brakes, the costs of the electronic control devices amount to a very large proportion of the total production costs of the dynamometer brake. Thus, in many instances smaller brakes and brakes already in operation have not been provided with a control mechanism at all in view of the high costs.
Further, in connection with brakes of the type described herein it has been noted that the filling condition of the dynamometer brake and the input power were subject to slight changes without any noticeable external influences. Such changes were particularly observed in the lower half of the power range of the hydraulic dynamometer brake and at rpms in the upper quarter of the rpm range of the respective brake. Such changes are beleived to be due to turbulent flow conditions in the area of the exit port of the hydraulic dynamometer brake.